In recent years, lightening of the aforenoted types of structures has been especially desirable, and a reduction in the thickness dimensions of the structures has accordingly been promoted in an effort to achieve this objective. Soft steel, stainless steel, aluminum alloys, and the like, are usually applied as outer plates for such panels. With the aforenoted promotion of the reduction in the thickness dimensions of the panel structures, however, the occurrence of "sag" within the outer plate member, after completion of the assemblage and welding operations of the outer plates to the underlying structural or support beams, inevitably arises, and problems are therefore posed with respect to the local strength of the "sagging" part of the panel as well as the external appearance of the panel.
With the intention of eliminating the aforenoted distortion effects of the welding process, there has accordingly been disclosed a technique wherein a large number of sheets of standard shape are formed into an elongated outer plate by means of a welding operation, whereupon under the state in which the outer plate is loaded or subjected to a stressed or tensioned state, such plate structures are then attached to a skeleton or structural framework, as disclosed, for example, within the official gazaette of Japanese Patent Application Publication No. 53-39261; or a technique wherein the aforenoted elongated outer plates is loaded in a tensioned state or with tensile stress and is also preheated so as to thermally expand the same, whereupon the outer plate can be attached to the skeleton or structural framework, as shown, for example, within the official gazette of Japanese Patent Application Publication No. 54-20185. Both of these techniques, however, require a large-scale tensioning and heating system which involve enormous installation costs as well as a large number of process steps.
Still further, another drawback of such system is that it is very difficult to precisely control the system so as to achieve a predetermined heating temperature.
Yet still further, there has also been provided a technique wherein an outer plate of a comparatively small size is disposed within a heating box or oven, and the entire outer plate is heated to a predetermined temperature so as to be thereafter welded to a skeleton or structural framework as shown for example within the official gazette of Japanese Patent Application Laid-Open No. 60-64791. In accordance with this technique, however, the heating box or oven is quite complex in structure, and moreover, as the larger plate becomes larger in size, the heating box or over necessarily needs to be proportionally larger in size so as to accommodate the larger plate and panel structure, whereby problems such as, for example, a large floor space and a workshop of large area are incurred, and in addition, the installation costs become significantly high.